1. Field of the Invention
The present invention relates to a zoom lens, which is suitable for an image pickup apparatus, such as a digital still camera, a video camera, and a TV camera.
2. Description of the Related Art
In recent years, an image pickup apparatus (camera) using a solid-state image pickup element, such as a video camera and a digital still camera has been downsized and developed to have high-performance. Then, due to the downsizing and high-performance of the image pickup apparatus, an optical system used therefor is required to include a wide field angle and to be a small zoom lens with a high zoom ratio, a large aperture ratio, and high optical performance. This type of camera includes various optical elements such as a low pass filter and a color correction filter disposed between the lens rearmost part and the image pickup element. Therefore, the zoom lens used for the image pickup apparatus is required to have a relatively long back focus. Further, in the case of a color camera using an image pickup element for a color image, the image pickup element is expected to have good telecentric characteristic on the image side so as to avoid color shading.
There is known a negative lead type zoom lens in which a negative refractive power lens unit is disposed closest to the object side, as a zoom lens with a small size of the entire system having a long back focus and good telecentric characteristic on the image side. As the negative lead type zoom lens, there is known a three-unit zoom lens having good telecentric characteristic on the image side, which includes, in order from the object side to the image side, a first lens unit having a negative refractive power, a second lens unit having a positive refractive power, and a third lens unit having a positive refractive power (Japanese Patent Application Laid-Open No. S62-200316, U.S. Pat. No. 6,124,984, U.S. Pat. No. 7,212,349).
In order to downsize the entire system of the zoom lens while achieving a high zoom ratio in a negative lead type three-unit zoom lens, it is effective to enhance a refractive power of each lens unit constituting the zoom lens. In addition, in order to realize a compact image pickup apparatus, it is effective to adopt a so-called retractable type, in which the interval between lens units is reduced during the non-image-taking period to an interval different from that in the image-taking state so as to reduce an extending amount of the lens from a main body of the image pickup apparatus. However, if the refractive power of each lens unit is simply enhanced for realizing a wide field angle and a high zoom ratio with a small size of the entire system of the zoom lens, for example, aberration variation due to zooming increases so that it becomes difficult to obtain high optical performance over the entire zoom range.
Other than that, if a movement amount of each lens unit during zooming and focusing is large, the total lens length is increased. Therefore, in order to obtain a desired retractable length, a complicated lens barrel structure is required. In general, the movement amount of each lens unit has a large correlation with the zoom ratio, and hence the above-mentioned tendency becomes conspicuous as the zoom ratio increases. In addition, in order to achieve a large aperture ratio, the lens unit having an aperture stop is increased in size so that the diameter of the marginal light beam increases. Therefore, it is necessary to increase the number of lenses or to take other similar countermeasures in order to correct the aberration sufficiently. If the number of lenses constituting each lens unit is increased, the total lens length in the retracted state increases so that it becomes difficult to downsize the camera.
Because of those reasons, it is important in the negative lead type three-unit zoom lens to set appropriately the lens structure of each lens unit in order to downsize the entire system of the zoom lens while achieving a wide field angle and a high zoom ratio as well as a desired retractable length. For instance, if a wide field angle and a high zoom ratio are to be attained without appropriate setting of the lens structure of the second lens unit and movement conditions of the first lens unit and the second lens unit during zooming, a front lens effective diameter increases at the wide angle side, and the size of the entire zoom lens increases.